In order to find any missing terms in an arithmetic sequence, you need to know the common difference, d, that is being added to get from one term to the next in the list. In order to find this, count how many missing terms there are between your two given values, and use those to solve for d. (Lower given number plus so many counted d's equals higher given number.) From there, you can solve for any term number in the list, including the fist, using the arithmetic explicit formula: nth term = first term plus d(n - 1).